The demand for biosensors is increasingly growing these days. Usually, biosensors allow for the detection of a given specific molecule within an analyte, wherein the amount of said molecule is typically small. Therefore, target particles, for example super-paramagnetic label beads, are used which bind to a specific binding site or spot only, if the molecule to be detected is present within the analyte. One known technique to detect these label particles bound to the binding spots is FTIR. Therein, light is coupled into the sample at an angle of total internal reflection. If no particles are present close to the sample surface, the light is completely reflected. If, however, the label particles are bound to said surface, the condition of total internal reflection is violated, a portion of the light is scattered into the sample and thus the amount of light reflected by the surface is decreased. By measuring the intensity of the reflected light with an optical detector, it is possible to estimate the amount of particles bound to the surface.
Typically, a photodiode is used as an optical detector. However, a detection by a (CCD) camera or any other multi-pixel system is much more efficient, since a camera allows for the parallel detection of various binding sites or spots. The disadvantage of the use of a camera, though, is that accurate measurements of the intensity of the binding spots is difficult due to gain —and offset errors in the read-out system.